Process for the manufacture of superphosphate



J 1938. s. G. NORDENGREN 2,106,223

PROCESS FOR THE MANUFACTURE OF SUPERPHOSPHA'IE Filed April 4, 1935Sulfuflc. Phosphate CL r: :11.

Fari'of- Phosphule Wed of Phospl'ufl'e Sulfuric Acid phosphoric acidCCLCLLUTI suljafe Insoluble mclHcr B Sf'age mLx er Producl"superphosphfie Phospho+e SuHuricAgid phosphonc OCLCl Calcium sulfa-FeProdud 4 Insoluble m Her fisu perphosphm e ATTORNEY Patented Jan. 25,1938 UNITED STATES PROCESS FOR THE MANUFACTURE OF SUPERPHOSPHATE SvenGunnar Nordengren, Landskrona, Sweden, assignor to Aktiebolaget KemiskaPatenter,

Landskrona, Sweden Application April 4, 1935, Serial No. 14,628 InSweden April 6, 1934 7 Claims.

This invention relates to a process for the manuiacture ofphosphoric-acid-containing fertilizers from a phosphoric-acid-containingmaterial and sulphuric acid with intermediate forma- The tricalciumphosphate is converted by the sulphuric acid to monocalcium phosphateand calcium sulphate.

The above described transformation proceeds in two individual reactions,which are partially superimposed. In the first reaction the sulphuric 2oacid acts upon a part of the tricalcium phosphate with formation ofcalcium sulphate and phosphoric acid:

(2) 208.3 (P04) 2+6H2SO4=6CaSO4+4H3PO4 The phosphoric acid so formedacts upon further quantities of tricalcium phosphate with formation ofmonocalcium phosphate:

(3) Ca: (P04) 2+4HsPO4=3CaH4 (P04) 2 At the same time small quantitiesof dicalcium phosphate are formed. A small part of tricalcium phosphateremains undissolved and the corresponding quantity of phosphoric acid(from 5 Equation 2) remains behind in the product in the form of freephosphoric acid. The sum of the two Equations 2 and 3, however,corresponds substantially to the course of the reaction taking place inthe production of superphosphate (Equa- 49 tion 1).

In the production of double superphosphate the decomposition reactionis' divided into two parts, phosphoric acid and calcium sulphate beingfirst produced, the latter separated, and the 45 resulting phosphoricacid employed for the decomposition of fresh quantities of rawphosphate.

The several steps of the present invention have been illustrateddiagrammatically in the accompanying drawing, in which, Fig. 1 is adiagram- 50 maticview of one mode of operation; and Fig. 2

is a diagrammatic view of another mode of operation.

According to the present invention the process is carried out in such away, that the reaction in 55 which phosphoric acid is formed, and thereaction which results in the formation of the phosphoricacid-containingfertilizer, are carried out for one part of the initialphosphoric-acid-containing material separately in regard to time andspace, without substantial quantities of the compounds formed by thereaction during the process of manufacture being separated. In the firststage of the process an excess of sulphuric acid is employed, and thisexcess is then directly converted into the end product in the secondstage.

In the first stage, therefore, i. e., in the production of thephosphoric acid, the process is carried out with a deficiency of aphosphoric-acidcontaining raw material, for example raw phosphate, i. e.with an excess of decomposition acid over the quantity necessary for theproduction of phosphoric acid, and in th'e'second stage acorrespondingly larger quantity of the raw material (phosphate) isintroduced, for example into a mixing screw device.

Thus, for example, the operation may with advantage be carried intoeflect by introducing in the first stage only about 30% of the phosphatecorresponding to the total amount of sulphuric acid, and making up thisdeficiency by introducing about 70% of the phosphate in the secondstage, the process being, accordingly, only separated with regard tospace and time with respect to a part of the total phosphate, thus inthe aforementioned case with respect to 15+45=60% of the total quantity.The remaining 55% are then directly converted to the end product in thesecond stage in the manner hitherto usually adopted in the art.

This method of operating has proved to be particularly advantageous incases, where it is desired to employ more concentrated sulphuric acidthan hitherto, and accordingly a somewhat more viscous paste is obtainedin the first stage. This method of operating further proves to beadvantageous particularly in the case of phosphates which are relativelydifficult to decompose.

With respect to the hitherto customary onestage method of producingsuperphosphate the present process ofiers a great number of advantages.Thus in the first stage the decomposition is carried out rapidly andthoroughly, because it can take'place in aliquid or relatively liquidme- .dium. Moreover, in the first stage relatively coarse calciumsulphate crystals are also formed, 50 which may then serve in the secondstage as a basis or nucleus for the freshly formed crystals of calciumsulphate.

It has been further found, that the advantages of carrying out thedecomposition in stages as 55 1 portions.

carrying out in stages the reaction in .whichfree phosphoric acid isfirst formed, or the reaction 'in which this phosphoric acid isconverted bymeans of further raw material, for example raw tions. Thus,for example, the'operation may be carried into effect by adding thequantity of raw phosphate which is required for the first stage, inwhich free phosphoric acid is formed, to the decomposition acid inseveral individual portions, instead of adding the entire quantity atthe start. A similar procedure may also be adopted in the second stageof the process, in which the mixture of phosphoric acid and the salt ofthe decomposition acid, for example calcium sulphate, obtained in thefirst stage, is allowed to react with fresh raw phosphate in order toobtain the final product. If, for example, the production 'ofsuperphosphate is regarded as described by formula in the initiallymentioned formula, the following reaction will take place according tothe described method of operation:

The quantity of 2 mols of tricalcium phos phate, necessary according toEquation 2, is not immediately mixed withthe 6 mols of decompositionsulphuric acid, but in several successive A similar procedure may befollowed in the second stage, which proceeds in accordance with Equation3, by introducing the quantity of tricalcium phosphate which is to reactwith the phosphoric acid formed in the first stage, viz: 1 mol. ofCa3(PO4)2 per 4 molsof H3P04, in successive individual portions, insteadof in one portion at the start.

Such a method of operating is accompanied by various special advantages.The decomposition is above all improved thereby. The first portions ofthe phosphate in Equation 1 come into contact with a larger quantity ofdecomposition acid and the formation of crust, for example of calciumsulphate, on the phosphate grains is thus to a great extent avoided.Further, the subsequent portions of phosphate, for example in theproduction of superphosphate, contact with already formed calciumsulphate crystals, which serve as crystallization nuclei. A furtheradvantage is a better elimination of acid gases, such as carbon dioxideand hydrofluoric acid compounds. In the second stage also, the firstportions of phosphate contact with more acid (phosphoric acid) than whenthe entire quantity of phosphate is added, whereby the decomposition islikewise facilitated. I

The effect of all these special advantages is, that at the end of thesecond stage a very satisfactorily decomposed and relatively dry productis obtained, since in the described method of operating the evaporationof water is also promoted. The physical form is also particularly good,i. e. the product is decidedly granular and particularly satisfactorilystrewable and contains a larger quantity of utilizable phosphoric acidper unit weight and volume than the products formed by the ordinaryprocesses of manufacture.

According to a particular embodiment of the invention the operation mayalso be carried out in such a way, that in the first stage thephosphoric-acid-containing raw material is brought into reactionsubstantially only with phosphoric acid, preferably by introducing theraw material into the upper part of the decomposition container and thesulphuric acid into the lower part.

If the phosphoric-acid-containing raw matedescribed above can-be stillfurther increased by rial, for example raw phosphate, in the first stagecomes into reaction. substantially only with the phate, which can then'in turn be subsequently 5 converted by the decomposition acid, forexample sulphuric acid, into phosphoric acid. By this method ofoperation the decomposing action and the quality of the end product canbe still further enhanced.

The same principle in sense of counter-current can also be carried out,for example, by performing the first stage of the process in a mixingscrew device, in the beginning part of which the phosphate is added, thesulphuric acid being added at a certain distance therefrom.

The above described method may be carried out in various types ofapparatus. Thus, for example, it is possible to use an apparatus, whichuses for the first stage several stirring containers, connected inseries, and for the second stage a mixing screw device. In each of thesecontainers a part of the phosphate necessary for the first stage isintroduced. The second stage may be then either carried out as describedabove in a mixing screw device, into which the entire quantity of thephosphate may be introduced at once, or this quantity may also beintroduced in portions at several spaced regions of the mixing screwdevice. The alternative procedure may, however, also be followed, whichconsists in carrying out the entire decomposition process, 1. e. thefirst and second stages, in one or more mixing screw devices, connectedin series, into which the phosphate is'i'ntroduced. in portions atdifferent places. Such an apparatus is particularly easy to superviseand control and takes up only a small space.

As compared with the former methods for producing superphosphate thisembodiment as well as the first described embodiment also offers the 4advantage of being capable of being operated continuously.

What I claim is:

1. A process of preparing superphosphate, which comprises mixing in afirst step phosphate 4 rock with sulphuric acid of relatively highconcentration in great excess over that required for the decompositionof all the phosphate rock to phosphoricacid, and thereupon reacting in asecond step the resulting unseparated mixture of 5 phosphoric acid,sulphuric acid, calcium-sulphate, and undecomposed phosphate rockresidue, with enough additional phosphate rock to formmonocalcium-phosphate and calcium-sulphate in the ratio found insuperphosphate. 5

2. A process according to claim 1, wherein the reaction of the firststep is divided into several stages by introducing the correspondingquantitles of the phosphate rock in portions and at different pointsinto the reaction mixture. 6

3. A process according to claim 1, wherein the reaction of the secondstep is divided into several stages by introducing the phosphate rock inportions and at different points into the reaction mixture. 6

4. A process according to claim 1, wherein each of the two steps isdivided into several stages by introducing the corresponding quantitiesof the phosphate rock in portions and at different points into thereaction mixtures. 7

5. A process asset forth in claim 1, in which the two steps are carriedout continuously during the advancement of the reaction ingredientsthrough at least one mixing device, the said phosphate rock beingintroduced into said device at two spaced regions thereof.

6. A process as set forth in claim 1, in which the two steps are carriedout continuously during the advancement of the reaction ingredientsthrough a plurality of successive mixing devices arranged in series, thesaid phosphate rock being introduced into said devices in separateportions.

'l. A process as set forth in claim 1, in which the two steps arecarried out continuously during the advancement of the reactioningredients through a plurality of two or more successive screw mixersarranged in series, the said phosphate rock being introduced into saidmixers in separate portions.

SVEN GUN'NAR NORDENGREN.

